Burn-in testing is a process used to screen early failures in electronic devices (e.g., semiconductor and integrated circuit devices) by operating the devices at elevated temperatures and elevated voltages over a period of time. This is accomplished by putting the devices under test (DUTs) in sockets on a printed circuit board (PCB) designed for such testing. These PCBs are usually referred to as burn-in boards (BIBs). BIBs are placed in an environmental chamber in a burn-in oven where they are connected to a current source for testing the operation of the DUTs. Basic burn-in testing may include providing only a clock signal to the DUTs to watch for the DUTs' responses thereto. Dynamic burn-in testing is a more sophisticated form of burn-in testing in that the burn-in test system has the additional capability to provide input stimuli to the DUTs. In such dynamic burn-in testing, clock and data signals exercise the device.
During burn-in, an electronic device is subjected thermally to both negative and positive feedbacks that affect the device's equilibrium temperature. The negative feedbacks are all parameters that cause an electronic device to cool down. These include the ambient temperature within an environmental chamber, if that ambient temperature is below the temperature of the electronic device, and the airflow in the environmental chamber.
Positive feedbacks include all parameters that cause an electronic device to heat up. One such parameter is thermal runaway. When a current source is connected to the BIBs with the DUTs in an environmental chamber during burn-in, the DUTs draw static current from the current source. “Static current” is the current that an electronic device draws when the device is turned on, but not in use. The amount of static current drawn by a device depends on device process, power up voltage, and temperature of the device. In addition to the static current, DUTs will draw dynamic current, if the burn-in is dynamic. “Dynamic current” is the current that an electronic device draws, in addition to static current, when the device is in use. The amount of dynamic current drawn by a device depends on how much of the total device is in use and how fast (clock frequency) it is being used. As the DUTs draw current from the current source, the DUTs produce heat, increasing the DUTs' temperature, which in turn causes the DUTs to draw more current. More current produces more heat and raises the temperature of the DUTs, causing the DUTs to draw still more current. This cycle, known as thermal runaway, can result in the temperature reaching the melting temperature of the DUTs. Thermal runaway results not only in the melting of the DUTs, but also the sockets in which the DUTs were attached during burn-in.
To minimize the risk of thermal runaway, burn-in testing has often been preceded by the sorting of the DUTs into groups of low, medium, and high static current devices. Each BIB is then used to burn-in only devices from a single group of devices at a temperature that minimizes the likelihood of thermal runaway for that device group. A drawback to this approach is that a device having a low static current may have reliability specifications that require a chamber environmental temperature that is higher than that which is used to test the device. This means that some devices cannot be tested at the temperature that is dictated by their reliability specifications. Another drawback to this approach is that it requires the additional test step of sorting the DUTs by static current level, a step involving a considerable amount of man and machine time that increases the production cost of the device.